I am building some free standing, perforated panel style helmholtz absorbers. These are built like cabinets with plywood front and back.

From what I know about helmholtz absorbers, they work best where sound propagation is at maximum pressure, i.e. close to a boundary. So it seems to me that the best place to drill holes would be in the rear of the cabinet where the panel is closest to the wall.

Is this assumption correct?

Here's a rendering of the basic concept, with perf panels in the front.

ah...
so much info regarding application is left unstated. And I apologize in advance as I suspect my response is not that for which you were hoping!

HH resonators are very effective if properly tuned and employed to address specific issues. But they are much less effective simply placed in a room. And along with location, they must be tuned to address a specific issue at that specific location.

Obviously I don' know your exact intentions here, but do you simply plan to place them in a room 'just to have them' with the thought that they will help ('something')?

I guess what I am saying is that I don't really understand your specific application and goal. And such information is a critical part of effectively designing (tuning) and employing tuned resonators - which, if such factors are effectively considered, can be very effective. But when just 'generally' employed, tend to be much less so.

Thanks for your reply.

I fully understand everything you said. I am employing helmholtz resonators to address very specific problems in this room. I have a pretty good understanding of general acoustics and tuned absorbers. I'm not simply building and placing them willy-nilly. They will be specifically tuned and placed to address specific problems in the room.

The attached drawing is, as stated, only a basic concept of how these things are being built; nothing more. In reality, the drawing has little to do with my question.

My question was actually very simple, and more general in nature:

Can you answer this simple question? Does placing the perforated panel of a helmholtz resonator closer to the boundary (where there is maximum pressure) increase the efficiency of the resonator?

Thanks again for any replies.

They are located at a point corresponding to the highest gain of the frequency to be absorbed. And since the antinode is the position corresponding to the point of highest gain and pressure, the absorber is most effective placed at such a point.

But herein lies a bit of a dilemma. If the resonator traps are sufficiently large, they modify the effective volume of the room and the effective boundaries and dimensions. Thus they become a part of, and their presence modifies, the very space they are themselves intended to address.

And as they are placed, the net effect is to modify the room response and the modal distribution slightly. Thus the tuning and placement requires a bit of iterative interactive in the tuning of the resonator, as the trap itself will slightly modify the room response that which it is addressing.

Likewise the maximum pressure for an antinode is at the boundary for the fundamental resonance frequency, while there will be additional antinodes distributed at intervals within the room for the harmonic multiples of the frequency. But while the pressure is greatest at the boundaries, the velocity of the wave will approach zero at the boundaries.

But if a large resonant absorber is placed near a boundary, the trap itself becomes a boundary of sorts. It is not independent of the space which it operates. Thus the wall is not the only boundary, but the trap itself must be accounted for within the system. Thus it is not so easy to state where the perforations should be located while failing to account for the interactive nature of the trap itself with the space in which it functions.

Thus, what you assume to be a "very simple question" is anything but!

It is also a reason that many tuned resonators are incorporated into the boundary surfaces of a space. And while such calculations of the complex system are exactly that - complex - it is also a reason that measurements are generally relied upon as they quickly and accurately indicate the actual effect the introduction of such tuned devices have upon the system in which they are designed to operate.

And if you stop and look at the very nature of the tuned trap you imagine - a Large bookshelf module - one can easily see how it can modify the effective room boundaries. And to the degree that they become an effective component of the bounded space, the openings are generally located in the surface of the resonator that effectively becomes an internal boundary.

...Sorry if this is not the "simple" answer you hoped for, but then the actual lumped parameter model is not quite so "simple" as many imagine.

Roger that. They will be placed based on modal distribution.

Comprendo. Hadn't considered that aspect. But of course that makes perfect sense.

Yes, this is the planned approach.

This is at the core of my question. What is the trade-off in pressure vs. velocity as far as a resonant absorber's efficiency is concerned. Is velocity required for the resonator action or is pressure all that matters?

Actually, it still seems pretty simple to me.

Yes, that is the planned approach.

That's the answer I was looking for. And it really was pretty darn simple!

Thanks!

Simple? In concept, sure!
...So is the Space Shuttle...and a heart transplant.

The calculations in order to avoid iterative placement and tuning are not.

What you have is the interaction of two coupled spaces. And you will pardon me if the reference to such a calculation as being simple cause me to smile.

Especially if one already assumes that a modal calculator is sufficiently accurate for ascertaining the behavior of a real room and uses those results to plan and design a tuned resonator that is further modified by the addition of additional coupled spaces...

If only the ACCURATE predictive calculations were as 'simple' as stating the issue!

Oh, and then you have the interaction of the various resonators to consider! It just keeps getting simpler and simpler! With all due respect, its "simple" only if you are willfully ignore-ant of the complexity of the variables involved. Achievable - yes; simple - no.

We will be using actual measurements to determine placement and tuning.

I really do appreciate your presence and input SAC! I always enjoy reading your responses on this forum. Thanks so much for playing along.

And BTW, the Space Shuttle actually doesn't seem that simple in concept to me.

So I'm still curious about the relationship between velocity and pressure as it applies to resonant absorbers (which was actually my original question).

Space travel became very 'simple' when I learned all I needed to know while studying for several degree in physics.

If you throw something up, unless you throw it hard enough, it will come down...
And if you get too close to the sun you burn up, and too far away, you freeze...

It was at that point that all of that mechanics and thermodynamics stuff become very 'simple' to me....hehhehheh

But a bit more seriously, as long as you realize that the process of designing and placing the resonators is iterative, and that while preliminary calculations are but the beginning, and that you NEED to rely on the iterative measurements to fine tune their application, you should do just fine.

The larger point that many miss in the application of such devices is that they cannot simply rely on an overly simplified predictive calculation for the final results.

Quote:

Originally Posted by Magickman ➡️

So I'm still curious about the relationship between velocity and pressure as it applies to resonant absorbers (which was actually my original question).

Resonate absorbers do not operate on the principle of simply dissipating energy (in the same manner where velocity is important) as do porous absorbers. While energy is dissipated as heat, the fundamental mechanisms are different - hence the resonant component..

While passive porous absorbers are dependent upon velocity, tuned resonators are dependent primarily upon pressure.

The HH design uses two forms of simple harmonic resonance: pistonic diaphragm resonance and Helmholtz cavity resonance.
In a HH resonator, the 'fluid' (in this case air, a 'compressible fluid') in the neck is the inertia element, while the 'fluid' in the cavity is the compliant component, and sound absorbing material placed in the neck and/or cavity is the dissipative component.

I might suggest looking at the various equations (or even better, a mechanical analogue diagram of the system) defining resonate absorbers and analyzing the role of the various variables to understand more fully the 'critical' 'functional' contributory aspects of their behavior.

Thanks. I have been studying D'Antonio and Cox for quite some time now (I actually pre-ordered the book when it was first published). Been trying to wrap my head around the math for a few years now. I just keep coming back, and it gets a little easier each time I do. That's how I arrived at my "velocity vs. pressure" question to begin with.

Incidentally, I never meant to imply that the actual predictive calculations were simple.

Actually, I would agree that the concept is exceptionally simple. ...Just let a child play with a Coke bottle by blowing across the opening for a conclusive demonstration!

But what both scares many away, and also causes much frustration and unfortunate abandonment of the technique by so many is that they desire a cookie cutter procedure whereby they can simply cut and assemble a tuned enclosure, stick it in the room, and have it behave in a manner ignoring the fact that it is but a piece in a complex coupled space system.

And if one is not willing to use measurements, the process can quickly become very frustrating as one cannot 'see' the deviation and the necessary iterative adjustments that are required for optimal performance. And hence why I suspect so many quickly run away and subsequently denigrate the technique.

But with a little perseverance, and the use of a few now readily available tools, they can be used to great advantage!

Good luck!

Keep in mind that most of us have limits on our time. I agree with what you've
just said and it's the primary reason that I don't consider Helmholtz resonators
as a useful solution. Especially wooden slat/slot walls. Imagine putting up a
wall only to find out that the frequency doesn't match the problem (a very
likely outcome I believe). Do a few compensating computations, tear everything
down, put it all back up, measure, tear it all down again... until you get it right.

Or in Magickman's case make a bunch of modules, install them, measure,
tear them down, redrill a few thousand holes, try again, etc...

I have a feeling that most of us would balk at entering into such a process.
But I can believe the results could be worthwhile.

Paul P

Actually, the way I've designed these units makes it very simple to perform iterative measurements and tuning.

First of all, the cabinets are free standing, like a bookcase. Each unit consists of number of sealed chambers, and the front each chamber is fitted with a removable plywood panel "cover". Each chamber is roughly 18 inches by 36 inches. To re-tune, I simply pull out one plywood panel, and substitute a different one. Each chamber can be tuned differently if required.

So the cabinets can be easily moved, and the perforation percentage easily changed.

Unless you have a quick way of perforating panels that could be time consuming.
But at least the rest doesn't change.

With respect to your original question, it seems to me that if you spaced your
modules apart and off the wall then bass frequency/pressure could get at
the back of your resonators and then if your holes are at the rear they
would be closer to max pressure. But if you stick your modules together
and up against the wall (?) like you propose they might be less effective.

I hope you implement your idea as I'd like to see how effective it is for my
own purposes.

Paul P

Paul, what you suggest is NOT the only way to adjustably tune a resonator.

But if you want to do it the hard way and only postulate Draconian steps, you are of course free to do so.

The D'Antonio & Cox text covers a complete gamut of tuned resonant absorbers in depth, and HH resonators are 'old hat' in mechanical noise control circles. I am sorry if the relationships are defined in terms of algebra, but hey, that's the language of mechanical behavior!

A little creativity in conjunction with an understanding of ALL of the variables can result in a few tunable alternatives that are actually used quite frequently. (Not to mention the relative ease o designing perforated or slotted tuned traps using any of a slew of readily available high tolerance perforated gratings available through metal supply catalogs.)

We already mentioned them a few months ago, but then everyone got hung up with the analogy regarding the material that was 'adjusted' and apparently completely missed the point!

But this is one process that I am not going to describe in detail here only because I know others who have worked quite hard to develop a strategic advantage in their business and I am, out of respect for them, not going to simply divulge all of the specific details.

But, that being said, with just a little thought and creativity, combined with a good understanding of the basic design principles, anyone should be able to effect a 'similar' system in very short order.

..............................

Also, the above system, when placed within the room, will 'become' a boundary! And as such it will modify the very space to be treated!!!!! In other words, the mode calculators only get you in the ballpark!!! You must then do measurements and iteratively modify the traps. But an astute designer knows this up front and thus needn't "tear apart" and rebuild!

Paul, my objection is that there seems to be much more effort put into explaining why something can't be done then there is in devising a pro-active plan to accommodate the an effective process to anticipate and address the various issues we know are involved!!

Here's another design I am playing around with. This is meant to sit in the front or rear floor/wall dihedral.

One drawing is an exploded view for clarity. Of course the would be some kind of absorptive just behind the "neck" of each hole.

Pretty simple design.

I've done this sort of thing before, and using a paper template, a center punch, and a drill press, it actually goes far quicker than one might think. I can drill one of the panels in question in pretty short order.

Oh, I'm definitely implementing it. The cabinets are already built!

I've extensively documented the whole process and will post up a nice build diary once I've completed the room. Been working on it full time for about 6 months now!

Good luck! thumbsup

I just hope you don't allow subsequent iterative tuning modifications to result in irreparable emotional trauma!

It's going to take a lot more than that to convince me that I should invest
a considerable amount of time in something that has so little documentation.
I'm close to calling bullshit to anyone who declares they have successfully
treated a room with helmholtz resonators. Have you ? Let's see some proof !

I think it's worthwhile to point out that any treatment based on anecdotal
information runs the risk of not working at all or making things worse. I
believe you've said the same yourself.

Paul P

How do you know how many of these you need so solve your specific problems ?
And what are your problems for that matter ?

I don't think it's terribly useful to discuss the construction aspects of resonators
without including some connection to their acoustic usefulness.

I'm not being beligerent, I'm just looking for information that I can believe in.

Paul P

Yes, Paul. I have used HH resonators for both room treatment and for similarly impossible tasks called designing bass-reflex speaker enclosures. One is driven internally and the other is driven externally. The concept is the same. And conveniently neglect commercial products such as RPG's Diffusorbloc and the Flutterfree HH component. And while not a true HH design, RPG's resonant Modex corner traps and stated opinion regarding the (in)effectiveness of porous corner traps is also sure to confuse you even further.

Funny, few have difficulty devising variable volume test enclosures for speaker design/testing.

And as far as whether you are willing to dare do anything as drastic as trying to build and tune a resonant trap...it really doesn't matter.

But then simply simply sitting, fretting and lamenting that someone else does not do it for you as you doubt a centuries old proven concept that apparently has no nexus with "acoustic usefulness" is just a bit silly. And whatever you do, avoid bass-reflex speakers. But you are of course free to do nothing but complain...despite its invaluable usefulness.

Great. Now let's see some before and after test results.

I'm not asking anyone to do anything for me. I only ask them to back up
their boasts. Otherwise it's just hot air. Ever heard of science ?

Paul P

No, Paul. I have never heard of science.

But then science in predicated upon premise, experimentation and a refinement of the premise. And that takes time and effort. Something you state is a waste of valuable time.

But it is fun to read your post insisting that I provide you with measured results as you, in the same post, claim that you do not want me to do anything for you.

And in the spirit of your request, I will not do anything for you. Some of us HAVE put in the time and effort to learn this stuff, and we're not here simply demanding answers from others who have made such an investment in time simply to satisfy someone who insists that they have neither the time nor interest to do so themselves. There is simply is no compelling reason waste my time to do so.